1. Field of the Invention
The present invention relates to a method for purifying an aqueous suspension with a module comprising wavy hollow fiber membranes. Particularly, the present invention relates to a method for purifying an aqueous suspension, for example, tap water such as river water, lake water, marsh water and ground water; water for industrial use; waste water; secondary treatment waste water; industrial sewage; domestic sewage; human waste; sea water and the like with the module.
Further, the present invention is also directed to a bundle of wavy hollow fiber membranes, a process for producing the same, and a hollow fiber membrane module comprising the same.
2. Related Art
Heretofore, various methods for purifying aqueous suspensions such as tap water, water for industrial use, waste water, industrial sewage, domestic sewage, human waste, sea water and the like with a hollow fiber membrane have been known. In particular, a purifying method according to so-called external pressure filtration, in which raw water permeates in the direction from the outer to inner surface of a hollow fiber membrane, can secure a larger membrane area contributing to filtration per unit volume when compared to so-called internal pressure filtration, in which raw water permeates in the direction from the inner to outer surface of a hollow fiber membrane. Therefore external pressure filtration is advantageously used in the field wherein minimization of water production cost is required, for example, a water-treatment field such as turbidity removal for waterworks.
In the above method for purifying aqueous suspensions with hollow fiber membranes, suspended or organic materials having a size bigger than the pore diameter of the membranes, are hindered on the membrane surface to cause a so-called concentration polarization or form a cake layer. Moreover, the organic materials in aqueous suspension clog the pores of membranes or adhere to the inner network structure of membranes. As a result, the filtration flux upon filtering the raw water is decreased to about one tenth of that upon filtering pure water. Accordingly, there have been also known purifying methods in which physical wash of membranes is regularly practiced after the filtration in order to achieve a more stable filtration flux.
Specifically, it has been known to carry out, after a certain-term filtration, backward wash comprising feeding a part of filtrate in the reverse direction to the filtration, i.e., in the direction from the filtrate side to the raw water side (hereinafter simply referred to as back wash), air-scrubbing comprising supplying compressed gases and/or compressed air and raw water in the direction from the lower to upper part of the hollow fiber membrane module filled with water to shake fibers and discharge suspended solids accumulating among the hollow fiber membranes from the system, and the like. For example, Japanese Patent Application Laid-Open No. 60-19002 discloses a method in which a bubble generation nozzle is arranged on the side of or down the hollow fiber membranes in the hollow fiber membrane storage container, and a gas is injected therefrom together with back wash.
On the other hand, as a membrane which can be used for the above-mentioned purifying method, a reverse osmosis membrane, an ultrafiltration membrane, a microfiltration membrane, a gas separation membrane, a nanofilter, and a deairing membrane have been known. These membranes are not used alone but used in plural, i.e., in the form of a hollow fiber membrane module. The membrane module is prepared by mounting a plurality of the membranes in a module case, sealing at least one edge thereof with a thermosetting resin such as an epoxy resin, and cutting the bonded and fixed portion to open a hollow portion. Such a module is used in various fluid treatment fields, for instance, a reverse osmosis membrane module is used for desalination of sea water or brine, production of primary pure water of ultra-pure water, and concentration of fruit juice or milk; an ultrafiltration membrane module for collection of electrodeposition paints, production of pyrogen-free water, treatment of waste water, concentration of enzymes, final filtration of ultra-pure water, and turbidity removal from tap water or waste water; a microfiltration membrane module for turbidity removal from tap water or waste water, treatment of concentrated water, germ removal and purification of fermentation liquid, and fine particle removal from chemicals, a gas separation membrane module for steam removal, condensation of hydrogen, condensation or enrichment of oxygen, condensation or enrichment of nitrogen, and condensation of carbon dioxide; a nanofilter module for removal of agricultural chemicals or halogenated organic compounds; and a deairing membrane module for deairing of water and aqueous solution. The hollow fiber membranes per se have also been studied. For example, Japanese Patent Application Laid-Open No.64-22308 discloses the art using an external pressure filtration type hollow fiber membrane module wherein hollow fibers having wavy or spiral curls at least in a part thereof are mounted instead of the conventional straight hollow fiber membrane in order to prevent such a mutual clinging of the hollow fibers that hinders raw water from flowing toward the center of the module and to use almost all the hollow fibers mounted in the module for effective filtration.
We, the present inventors, have made studies on a purifying method enabling a stable filtration.
In particular, we have made studies focusing on the fact that the hollow fiber membrane surface is considerably damaged when the aqueous suspension comprising fine particles containing an inorganic component is purified according to an external pressure filtration method taking a step of physical wash such as back wash and air-scrubbing.
As a result, we have found that when an inorganic component is contained in the suspended solid accumulating among hollow fiber membranes, the outer surfaces of the membranes rub against each other through the suspended solids at the physical wash step and pores on the surface of the membranes are covered with the result that the stability of the filtration operation is deteriorated. Further, we have found that the continuation of such a phenomenon may result in the breakage of the membranes.
As the result of our extensive and intensive studies, we successfully provided, by using a bundle of hollow fiber membranes having specific waves, diameter and further bulkiness, a purifying method enabling a stable filtration, in which the damage of membrane surfaces caused by an inorganic component at the physical wash step, is unexpectedly decreased.
Moreover, in the production of a hollow fiber membrane comprising extruding membrane production raw liquid followed by cooling and then solidification or coagulation, we have also succeeded in efficiently producing a bundle having specific waves, diameter and further bulkiness as described above by contacting a pulsation flow with hollow fiber materials under specific conditions to vibrate and cooling and solidifying or coagulating the hollow fiber materials while vibrating.
Namely, it is an object of the present invention to provide a purifying method enabling stable filtration, in which, during the physical wash step following filtration of aqueous suspension, the external surfaces of hollow fiber membranes is prevented from rubbing against each other through suspended solids in the aqueous suspension containing an inorganic component, and the covering of open pores on the surface of hollow fiber membranes is hindered. It is also an object of the present invention to provide a bundle of the hollow fiber membranes which is used for the purifying method, a process for producing the bundle, and a module mounting the bundle.
It is another object of the present invention to provide a purifying method enabling a stable filtration, in which the efficiency of the physical wash to discharge suspended solids accumulating among hollow fiber membranes is improved without damaging the surface of the hollow fiber membranes. It is also another object of the present invention to provide a bundle which can be used for the purifying method, a method for producing the membrane bundle, and a module mounting the bundle.
Insufficient physical wash may cause accumulation of suspended solids in the hollow fiber membrane bundle so that the membranes cling to each other in the shape of a rod. In such a case, raw water cannot be supplied into the bundle with the result that the amount of the filtrate to be recovered is seriously decreased. Additionally, if physical wash is conducted in the state that the hollow fiber membranes are clinging to each other in a rod shape, the hollow fiber membranes may be broken by an excessive external force toward the horizontal direction The present invention also solves such problems.
It is another object of the present invention to provide a bundle capable of decreasing defects caused upon bonding and fixing a bundle to a module case even if a bonding agent to be employed has a high initial viscosity before hardening or the bundle is bonded and fixed to a large-scale module case with a large diameter, and to provide a process for producing the bundle.
The above-mentioned objects of the present invention can be achieved by the following.
1) A method for purifying aqueous suspension comprising feeding aqueous suspension containing a fine particle comprising an inorganic component from the outer surface of a wavy hollow fiber membrane having an outer diameter of from 0.5 to 3.1 mm to filter, followed by physical wash of the hollow fiber membrane.
2) A hollow fiber membrane bundle which is prepared by collecting a plurality of wavy hollow fiber membranes so as to orient in the same direction with a bulkiness of from 1.45 to 2.00, wherein the membrane has an inner diameter of from 0.3 to 1.7 mm, an outer diameter of from 0.5 to 3.1 mm, a membrane thickness of from 0.1 to 0.7 mm, and a flatness of from 0.8 to 1.0.
3) A method for producing a hollow fiber membrane bundle comprising the steps of:
i) extruding membrane production raw liquid in the form of a hollow fiber through a co-axial tube-in-orifice spinning nozzle to obtain a hollow fiber material,
ii) cooling and solidifying or coagulating the hollow fiber material to obtain a hollow fiber membrane, and
iii) collecting a plurality of the thus-obtained hollow fiber membranes so as to orient in the same direction;
wherein a pulsation flow is contacted with the hollow fiber material before or during the cooling and solidifying step or the coagulating step.
4) A hollow fiber membrane module, wherein a plurality of wavy hollow fiber membranes each having an inner diameter of 0.3 to 1.7 mm, an outer diameter of 0.5 to 3.1 mm, a membrane thickness of 0.1 to 0.7 mm and a flatness of 0.8 to 1.0 is collected so as to orient in the same direction and mounted with a packing ratio of from 35 to 55%.